Apparently prostate cancer has some dietary epidemiology which suggests that the cancer could develop in more than one way, from different starting points. In various populations studied, a number of dietary factors have been shown to affect the incidence of the disease, but not consistently across all populations studied. The factors included types and amount of fat, meat, beta-carotene, calcium, tomato lycopene, skim milk, garlic, forms of vitamin D, total increase in food volume over previous generations, as well as body fat and genetic factors and others.
Given the possibility of multiple starting points, at this stage in the research it would be hard to tease out how a blood type/immune system type might affect an outcome.
That does not preclude the possiblity that immune system type might be linked to particular sub-patterns.
My father's prostate cancer certainly fit some of the epidemiological profiles: he loved (1) skim milk and drank it because he wanted to have plenty of calcium (2) and lose excess weight (3) which had crept on him in a sedentary lifestyle. He seldom ate food with beta-carotene (4) or garlic (5) or fruits (6) or tomatoes with lycopene (7). Given the vitamin A and D boost in milk, he probably got large amounts of them in his diet.
Had he followed his blood type diet for many years, that profile would probably not have developed. But that could be true whether he was an O, A, B, or AB.
Something to mull over.